<bean id="multipartResolver"
class="org.springframework.web.multipart.commons.CommonsMultipartResolver">
<!-- set the max upload size100MB -->
<property name="maxUploadSize">
<value>104857600</value>
</property>
<property name="maxInMemorySize">
<value>4096</value>
</property>
<property name="defaultEncoding">
<value>utf-8</value>
</property>
</bean>
上述配置是springmvc框架下上传文件常见的配置,有了上面的配置,下面controller就可以很容易完成文件上传工作:
@PostMapping(value = "/upload")
public String upload(HttpServletRequest request){
logger.info("上传文件");
try {
MultipartHttpServletRequest multipartRequest = (MultipartHttpServletRequest) request;
MultipartFile file = multipartRequest.getFile("file");
//处理上传的文件
} catch (Exception e) {
logger.error(e.getMessage(),e);
}
}
注意上面标红部分代码,那么CommonsMultipartResolver工作的背后原理是什么呢,笔者带着这个问题将一探究竟:
DispatcherServlet::protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
processedRequest = checkMultipart(request);
DispatcherServlet:: protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
logger.debug("Request is already a MultipartHttpServletRequest - if not in a forward, " +
"this typically results from an additional MultipartFilter in web.xml");
}
else if (hasMultipartException(request) ) {
logger.debug("Multipart resolution failed for current request before - " +
"skipping re-resolution for undisturbed error rendering");
}
else {
try {
return this.multipartResolver.resolveMultipart(request);
}
catch (MultipartException ex) {
if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) != null) {
logger.debug("Multipart resolution failed for error dispatch", ex);
// Keep processing error dispatch with regular request handle below
}
else {
throw ex;
}
}
}
}
// If not returned before: return original request.
return request;
}
上面multipartResolver.resolveMultipart就会调用CommonsMultipartResolver的resolveMultipart方法,最后真正转换发生在类CommonsFileUploadSupport的parseFileItems方法
protected MultipartParsingResult parseFileItems(List<FileItem> fileItems, String encoding) {
MultiValueMap<String, MultipartFile> multipartFiles = new LinkedMultiValueMap<String, MultipartFile>();
Map<String, String[]> multipartParameters = new HashMap<String, String[]>();
Map<String, String> multipartParameterContentTypes = new HashMap<String, String>();
// Extract multipart files and multipart parameters.
for (FileItem fileItem : fileItems) {
if (fileItem.isFormField()) {
String value;
String partEncoding = determineEncoding(fileItem.getContentType(), encoding);
if (partEncoding != null) {
try {
value = fileItem.getString(partEncoding);
}
catch (UnsupportedEncodingException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Could not decode multipart item '" + fileItem.getFieldName() +
"' with encoding '" + partEncoding + "': using platform default");
}
value = fileItem.getString();
}
}
else {
value = fileItem.getString();
}
String[] curParam = multipartParameters.get(fileItem.getFieldName());
if (curParam == null) {
// simple form field
multipartParameters.put(fileItem.getFieldName(), new String[] {value});
}
else {
// array of simple form fields
String[] newParam = StringUtils.addStringToArray(curParam, value);
multipartParameters.put(fileItem.getFieldName(), newParam);
}
multipartParameterContentTypes.put(fileItem.getFieldName(), fileItem.getContentType());
}
else {
// multipart file field
CommonsMultipartFile file = new CommonsMultipartFile(fileItem);
multipartFiles.add(file.getName(), file);
if (logger.isDebugEnabled()) {
logger.debug("Found multipart file [" + file.getName() + "] of size " + file.getSize() +
" bytes with original filename [" + file.getOriginalFilename() + "], stored " +
file.getStorageDescription());
}
}
}
return new MultipartParsingResult(multipartFiles, multipartParameters, multipartParameterContentTypes);
}
该类将来自客户端上传的文件转换为FileItem对象,这样就完成将multipart/form-data到MultipartFile对象的转换,
因为spring默认不启用MultipartResolver的支持,所以使用前需要进行MultipartResolver
Bean的定义,但是springboot默认支持了MultipartResolver,直接就可以使用!